Most ASMR episodes begin by an external or internal trigger, and are so divided for classification. Type A episodes are elicited by the experiencer using no external stimuli, and are typically achieved by specific thought patterns unique to the individual. Type B episodes are triggered involuntarily by an external trigger, via one or more senses, and may also involve specific thought patterns associated with the triggering event. Both types of triggers vary between individuals, but many are common to a large portion of ASMR enjoyers.

Common external triggers:

Exposure to slow, accented, or unique speech patterns

Viewing educational or instructive videos or lectures

Experiencing a high empathetic or sympathetic reaction to an event

Enjoying a piece of art or music

Watching another person complete a task, often in a diligent, attentive manner - examples would be filling out a form, writing a check, going through a purse or bag, inspecting an item closely, etc.

Close, personal attention from another person

Haircuts, or other touch from another on head or back

My pal Mark Houston stumbled across an assortment of clips meant to induce this state of mind, and we couldn't stop watching them. I posted it to a friend in Australia who used to be driven mad by things like people who chew cereal loudly, and even she was oddly lulled.

Goosebumps are another one. Not sure if yawning is considered one or not, but sometimes I've noticed that after watching the same video over and over I'll yawn at specific points, not because of boredom or tiredness.